1. Field of the Invention
The present invention relates to a hydraulic pressure control apparatus having a relief valve, which is capable of being opened if the pressure of a working oil becomes equal to or greater than a predetermined threshold value.
2. Description of the Related Art
One known type of hydraulic pressure control apparatus is disclosed in Japanese Laid-Open Patent Publication No. 05-345528. The disclosed hydraulic pressure control apparatus serves to regulate the pressure of a working oil that is supplied to the clutch of an automobile. The hydraulic pressure control apparatus includes a differential clutch control valve and a solenoid modulator valve that function as pressure regulating valves, and a single linear solenoid-operated valve. The initial pressure (line pressure) of the working oil is reduced to a predetermined pressure by the solenoid modulator valve, and the reduced oil pressure is supplied to the linear solenoid-operated valve.
The linear solenoid-operated valve generates a solenoid pressure based on an electric current that is supplied to the solenoid thereof, and the solenoid pressure is supplied to the differential clutch control valve. Based on the solenoid pressure, the differential clutch control valve converts the line pressure, and the working oil is supplied to the clutch under the converted pressure, which acts as a predetermined clutch pressure (actuating pressure).
According to Japanese Laid-Open Patent Publication No. 05-345528, among the three valves that function as described above, it has been proposed that the differential clutch control valve and the solenoid modulator valve are arranged in parallel facing relation to each other within a valve body, which is made up of a combination of a lower valve body and an upper valve body, whereas the linear solenoid-operated valve is arranged between the two pressure regulating valves with the axis of the linear solenoid-operated valve extending perpendicularly to the two pressure regulating valves. Further, oil passageways for the working oil are defined in the upper valve body.